The patterns in an integrated are created by etching under a photoresist mask that was formed from a glass mask through a photolithographic process. The size of the minimum feature in an integrated circuit is usually referred to as the critical dimension (CD). As the CD approaches the wavelength of the light that was used to image the glass mask and expose the photoresist (between about 1830 and 3650 Angstroms for a Deep UV source), the patterns formed in the photoresist cease to reproduce the patterns on the glass mask with complete fidelity. Because the effect on any given feature is greatly influenced by the feature's surroundings, the phenomenon has been named `the proximity effect`.
FIGS. 1a-c illustrate three different manifestations of the proximity effect. In FIG. 1a line 2 is isolated and has no immediate neighbours whereas lines such as 3 are crowded together, being separated by a space that is comparable to their width. Although lines 3 and line 2 had the same width on the glass mask from which they were imaged onto the photoresist, the proximity effect has caused lines 3 to be narrower than line 2. In FIG. 1b, line 4 on the glass mask had a length corresponding to dimension 5 but, in the photoresist image, it was considerably shortened, as shown. In FIG. 1c, the rounding effect of a corner that was intended to be square is shown, photoresist being absent from the area marked as 6.
Although the origins of the proximity effect are understood, calculating its magnitude for any given pattern can be very complicated and time consuming. Nevertheless, it is currently the general practice of the semiconductor industry to perform such calculations in order to generate an Optical Proximity Correction (OPC) which can be applied to the original glass mask pattern to compensate for the anticipated optical proximity effects.
The process of transferring the glass pattern to a photoresist image can be broadly summarised into four steps: 1) resist coating, 2) exposure, 3) post exposure bake (PEB) and 4) development. The surface on which the resist is coated may or may not be an anti-reflection coating (ARC). This is relevant as the proximity effect will be influenced by (among other things) the degree to which standing wave patterns are formed within the photoresist layer. However, it turns out that steps 2 and 3 are where proximity effects are introduced and, furthermore, by carefully controlling the conditions under which these two steps are implemented, the proximity effect can be eliminated, thereby removing the need for the OPC and associated costly calculations.
During a search for possible prior art, several references were found to be of interest. These include Itoo et al. (U.S. Pat. No. 5,436,114 July 1995), Ootaka et al. (U.S. Pat. No. 5,636,004 June 1997), and Gortych et al. (U.S. Pat. No. 5,680,588 October 1997) all of whom discuss the importance of numerical aperture and/or coherency. Liu et al (U.S. Pat. No. 4,988,284 January 1991) teach the need for a Post Exposure Bake but this is for electron beam resists and, furthermore, a temperature of at least 100.degree. C. is specified.